1. Field of Invention
The present invention relates a power supply control circuit, and more particularly to an inrush current control circuit.
2. Related Art
Inrush current is a common problem with power supply circuits. It usually occurs at the moment a power supply is turned on, producing an immediate surge current, which may produce signal noise, damage power devices and connected equipment. To prevent inrush current, several circuit structures have been developed.
Conventionally, a current limiting resistor is used in inrush current control circuit to connect with the input circuit in series to limit the inrush current within a specified value. However, the series resistor requires a high resistance, which may lead to the waste of power and the generation of unnecessary heat when the system is in normal operation.
Referring to FIG. 1, a conventional inrush current control circuit includes a rectifying circuit BR, a capacitor C, and the main body of the system, wherein a negative temperature coefficient (NTC) thermistor RT is placed on a side of the alternating current (AC) power to control any possible inrush current. The idea is to make use of the characteristic of the NTC thermistor RT that the resistance value of the NTC thermistor is higher at low temperatures and lower at high temperatures. At the moment an electronic device is turned on or an alternative current (AC) power is supplied, the NTC thermistor is at low a temperature and thus has a high resistance, limiting the inrush current of the AC power. As the temperature of NTC thermistor goes up, its resistance begins to drop, and thus the whole system enters to the normal operation condition. However, this type of circuit structure still has problems such as high power consumption and being ineffective under warm boot.
FIG. 2 illustrates another type of conventional inrush current control circuit, in which a current limiting resistor R and a switch SW are placed at a side of the alternative current to limit the inrush current. When the power is supplied, the current limiting resistor R starts to limit current and the capacitor has not been charged yet. The current value at this time is high and may therefore cause damages to other elements. Then, after the capacitor C has been fully charged, the current value begins to drop. However, the current is still relatively high. As a result, the current limiting resistor R continues releasing large amount of heat. Finally, the switch SW, such as a relay, is turned on, and thus the current bypasses the current limiting resistor R to thereby prevent the efficiency of the whole system from being decreased due to the heat released from the current limiting resistor R. However, if the relay is constantly in an “on” state, it is likely to be damaged.
FIG. 3 illustrates still another type of conventional inrush current control circuit, wherein two switches SW1 and SW2 are used to control potential inrush current. The switches SW1 and SW2 are placed at the positive poles of the rectifying circuit BR and the capacitor C, and are controlled by voltage signals from the high voltage terminal of the system. In addition, the elements used in this type of circuit must be able to resist high voltage; therefore, the control circuit has complicated circuit and high cost for elements problems.
Moreover, there has another inrush current control circuit using a timing switch to control inrush current. When an AC power is supplied, a current limiting resistor provides the function to limit current. After the power becomes stable, the current will bypass the current limiting resistor. When the AC power is cut off, the timing switch will also turn off. Specifically, the voltage of the internal direct current (DC) supply rail will decrease; thereby the timing switch will turn off. This type of inrush current control circuit also has the same problems as the inrush current control circuits using NTC thermistors, that is, the timing switch needs a certain period of time to return to the “off” state. Thus, the timing switch should not be connected immediately after the AC power is cut off; otherwise if the current limiting resistor is bypassed, limiting the inrush current may not be realized.
As described above, there is more room for improving the design of inrush current control circuits.